String is not null forever?

Question

String a = "";
String b = new String();
a==null? false
b==null? false
why?

Answer 1

you mean a.b are Strings with length 0 but they are
exist so not equal Null?Not quite.
a and b are variables. They hold references. Those references could be null (which means they don't point to an object) but here they're not. They both point to String objects. Those String objects don't happen to contain any characters in their char arrays (the arrays themselves also exist, and have length zero), but the String objects still exist.
Objects (such as Strings) can never be null. That concept doesn't apply. Only references can "be null" (i.e., the value they hold is the null reference -- http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html#11128).
Remember the finger-pointing and paper-writing examples from your other thread? Setting a reference to null is making a fist and pointing at nothing, or erasing whatever is written on a piece of paper. It has nothing to do with any objects. Setting a variable to the empty string "" is pointing your finger at a book that has only a front and back cover, but no pages inside.